1. Field of the Invention
This invention relates to a control unit for a differential pressure switch (DPS) sensor and, more specifically, to such a control unit with a trip free, tamper proof magnetic reset.
2. Brief Description of the Prior Art
In general, control units for DPS sensors control the time duration that an improper condition, such as, for example, low pressure, can exist before the normally closed output contacts of the control unit are opened. Under normal operation, a motor contactor will be controlled by the control unit and will receive power while the output contacts are closed. This control unit must remain in the tripped condition (normally closed contacts open) until it is reset by some outside action, such as the pressing of a reset button.
Many control devices may be connected to the motor contactor in the same fashion as the DPS sensing control unit. Furthermore, all such serially connected controls must receive their power from the control lines of the contactor (in contrast to the motor, which receives its power from the contacts of the contactor). Thus, whenever any of these serially connected control units, including the DPS sensing control itself, opens its normally closed output contact, power is removed from the DPS control unit. THE DPS sensing control must remain in its tripped position until reset, even if power is removed and reapplied, for safety reasons. This implies the need for the DPS control to have retention or memory of the tripped condition. The DPS sensing control must also be resettable without power applied thereto. Otherwise, all serially connected controls would have to be reset before resetting of the DPS control unit. Finally, for better protection/safety, a DPS control unit must eliminate the possibility of tampering with the reset mechanism to make the normally closed contacts stay closed.
Prior art for DPS sensing control utilized capacitors to retain the tripped information or batteries. Still another prior art means of retaining the tripped data was to store the energy in a snap acting bimetal disc. Some other methods required that power not be removed from the DPS sensing control following a trip condition. The problem with batteries is their short life at high temperatures. Capacitors, on the other hand, can store energy for only a short period of time. In addition, bimetal devices are prone to variations in supplied power and require a delay time before they may be reset. Almost all of the prior art devices use a reset button, however none incorporate any antitampering techniques.